We analyzed the cell surface phenotype and the function of mononuclear cells in peripheral blood and in bone marrow of patients with rheumatoid arthritis (RA). The monoclonal antibodies anti-CD45RA, anti-CD29 and anti-S6F1 identify the suppressor-inducer (CD4+CD45RA+), helper-inducer (CD4+CD29+) and killer-effector (CD8+S6F1+) subpopulations of lymphocytes, respectively. In patients with RA, peripheral blood samples showed the same percentage of CD4+CD45RA+, CD4+CD29+ and CD8+S6F1+ cells as seen in control subjects. In contrast, in the bone marrow of patients with RA we observed a significant decrease in CD4+CD45RA+ cells, a significant increase in CD8+S6F1+ cells compared with findings in peripheral blood and in bone marrow samples from control subjects. Consistent with the phenotypic changes observed, bone marrow T cells also showed functional abnormalities, since autologous mixed lymphocyte reaction-activated CD4 cells from bone marrow of patients with RA showed a decrease in suppressor-inducer activity and CD8+ cells activated by allogenic E- cell showed an increase in killer-inducer activity. The changes noted above may contribute to the immunologic abnormalities that occur in this disease and provide insight into the pathophysiology of RA.